Many non-invasive or minimally invasive medical procedures employ medical guidewires to direct a catheter or a mechanical medical device to a desired treatment site within a patient's vascular system or the like. When performing such procedures, the operator typically views the progress of the guidewire within the patient's body by means of a fluoroscope or other radiographic instrument. By watching the guidewire on the monitor and manipulating the proximal portion of the guidewire outside the patient's body, the operator can guide the distal portion of the guidewire to a position adjacent the treatment site. Once properly positioned, a catheter or other treatment device, e.g. an atherectomy device, can be guided over a portion of the guidewire and the desired treatment can be performed.
Some materials are more radiopaque than others, with the more radiopaque materials being easier to see on the monitor than the less radiopaque materials. The majority of the guidewires commercially available are made of stainless steel or other metals having a similar radiopacity. Stainless steel is not very radiopaque at smaller diameters, though, and most stainless steel guidewires below about 0.032 inches in diameter are provided with a radiopaque area adjacent the distal end to make the distal end of the guidewire more visible for deployment. This radiopaque area may take the form of radiopaque marking bands, for example, which typically are small rings of a more radiopaque material such as gold, platinum or tungsten, attached to the guidewire.
Many guidewires include a helically wound coil carried over a distal segment of the guidewire to increase flexibility of the distal tip. A core wire usually extends along the length of the lumen of the coil and a bead is provided at the end of the guidewire. The bead is attached to both the core and the coil to lock them together at the distal end. This will help prevent a segment of the guidewire from being lost in the patient's body if the coil or core breaks and also help transmit torque to the coil when the proximal part of the guidewire is turned by the operator to steer the guidewire.
Some guidewires use integrally formed core wires which extend throughout the entire length of the coil and are either bonded to or integrally formed with a bead on the distal end. Other guidewires include a "safety wire" which is bonded to the distal tip of the main wire of the guidewire, which terminates within the lumen of the coil, and the safety wire extends distally to be bonded to a bead at the distal end of the coil. In guidewires which use safety wires, the safety wire must be securely bonded to the core wire, typically by means of a solder or braze joint.
One way which has been used to make stainless steel guidewires more visible is to make the helical coil adjacent the distal end of the guidewire more radiopaque. This may be accomplished by forming the coil from wire of a radiopaque material or by coating the coil with a relatively thick coating of a radiopaque material, such as coating the coil with gold or platinum. Since radiopacity is tied, at least in part, to density, it is generally easier to see a coil which is formed entirely of a radiopaque material than it is to see a coil coated with the radiopaque material.
Although making a coil carried over a distal portion of the guidewire from a radiopaque material makes it easier to steer the guidewire because it is easier to see, this increased visibility can interfere with visualization of the patient's tissues to be treated. Some tissues being treated by the operator tend to show up fairly poorly on the monitor. Other tissues to be treated, such as a calcified atheroma in a patient's vessels, tend to be fairly radiodense. When a solid, radiopaque length of the guidewire is positioned in the same area of the monitor view as the tissue to be treated, it can frequently be very difficult to see the tissue adjacent to or surrounding the guidewire--if the tissue is not very radiodense the guidewire can essentially mask the treatment site, while if the tissue is more radiodense it can be hard to distinguish the guidewire from the tissue, making it difficult to locate the margins of the treatment site.
Some have proposed solving this problem by making the helical coil of the guidewire of two different materials. For example, U.S. Pat. No. 4,538,622 (Samson et al.) shows such a two-material design. In such designs, a distal segment of the coil is formed of a strong radiopaque material, such as gold or platinum, and the rest of the coil is formed of a less radiopaque material, such as stainless steel.
Such a guidewire design permits the operator to easily see the distal coil length during deployment of the guidewire. The guidewire is urged past the treatment site so that the distal coil segment is positioned beyond the tissue to be treated. This positions the less radiopaque proximal coil segment adjacent the treatment site, permitting the operator to see the tissue at the treatment site with minimal interference in visibility from the guidewire itself.
Although such two-material coils do have certain advantages, they tend to be more difficult to steer into position than some other guidewires. Such guidewires can also present problems in tracking a catheter or other treatment device along the guidewire adjacent the junction between the proximal and distal coil segments. The distal and proximal coil segments typically must be bonded to one another and to the core wire extending through the lumen of the coil to provide a sure connection and to ensure that torque of the proximal segment of the guidewire is transmitted to the coil to steer the distal segment of the wire.
Samson's soldered design can make it difficult to steer the wire through a patient's vascular system. This problem is particularly acute when the guidewire is used in narrowing vessels, such as vessels which are narrowed by atheroma. In Samson's design, if movement of the coil is restricted, such as when the coil is in contact with atheroma, the friction of the coil against the patient's tissue can make it very difficult, if not impossible, to turn the tip of the guidewire to steer it beyond the narrowing of the vessel because the intermediate solder bond to the coil prevents, or at least greatly restricts, relative movement between the core wire and the coil.
The junction between the three elements (or four elements, if a safety wire is used) of Samson's design yields a stiff, hard length of the guidewire which can often be a centimeter or longer. It has been found that sharp discontinuities in flexibility, however, tend to cause the guidewire to bend unevenly during deployment, which can make the guidewire more difficult to steer. When a catheter or other device is urged forwardly over the guidewire for treatment, it will follow, or "track", the guidewire into the treatment position. If the guidewire defines fairly smooth, continuous curves as it bends, it will be fairly easy for catheters to track the guidewire. When the guidewire has relatively sharp discontinuities in flexibility, this will interrupt the otherwise smooth flow of a curve, making it more difficult to track a catheter or the like over the guidewire.
Accordingly, two-material coils such as those proposed by Samson et al. are useful in providing enhanced visibility for deployment without significantly interfering with an operator's ability to see a patient's tissue during treatment. However, these designs are inherently flawed in that they tend to produce sharp discontinuities in flexibility, making it more difficult to steer the guidewire into place and to deploy the treatment device.